This invention relates to an elastic surface wave gyroscope for detecting a Coriolis force generated on the surface of a piezoelectric substrate by the interaction of a surface oscillation caused by an elastic surface wave of the piezoelectric substrate and a rotary motion of the piezoelectric substrate by converting it into a voltage.
A known gyroscope which uses elastic surface waves is disclosed in, for example, Japanese Unexamined Patent publication No. 6-281465.
Specifically, this publication discloses an elastic surface wave gyroscope constructed such that a pair of inter-digital transducer for generating elastic surface waves of the same frequency (hereinafter, "driving IDTs") with an inter-digital transducer for detecting a Coriolis force (hereinafter, "detecting IDT") arranged therebetween ,and a pair of reflectors provided on the outsides of the driving IDTs for reflecting elastic surface waves toward the detecting IDT are so formed on one surface of a piezoelectric substrate to have a specified positional relationship to each other.
FIG. 21 is a diagram showing the detecting IDT, the pair of driving IDTs and the pair of reflectors formed on the surface of the piezoelectric substrate of the elastic surface wave gyroscope.
Distances d1, d2 between teeth of comb-shaped electrodes D1, D2 of the detecting IDT 101 and the driving IDTs 102 formed on the surface of the piezoelectric substrate 100 are equal (1/2 of a wavelength .lambda. of the elastic surface wave). The reflectors 104, 105 are constructed by grating radiators in which 100 line electrodes D3 are arranged at specified intervals (approximately .lambda./2).
In the piezoelectric substrate 100, standing waves of elastic surface waves are generated by causing the respective driving IDTs 102, 103 to generate elastic surface waves propagating in outward directions therefrom and by reflecting these elastic surface waves toward the detecting IDT 101 by the reflectors 104, 105. The detecting IDT 101 is formed such that the teeth of the respective comb-shaped electrodes D1, D2 are located in predetermined positions corresponding to the nodes of the elastic surface wave (standing wave).
In the above elastic surface wave gyroscope, when the piezoelectric 100 undergoes a rotary motion with the standing wave of the elastic surface wave generated on the surface thereof, an elastic surface wave (standing wave) is generated caused by a Coriolis force and phase-shifted by 90.degree. from this elastic surface wave along the vertical direction with respect to a direction of oscillation cause by this elastic surface wave. Accordingly, the detecting IDT 101 detects a voltage corresponding to the oscillation of the elastic surface wave converted by the piezoelectric effect.
The above elastic surface wave gyroscope generates the standing elastic surface waves between the driving IDTs 102, 103 by reflecting the elastic surface waves of the same frequency generated by the driving IDT 102, 103 toward the detecting IDT 101 by the reflectors 104, 105. Since the Coriolis force generated by the interaction of the elastic surface wave and the rotary motion of the piezoelectric substrate 100 is phase-shifted by 90.degree. from the elastic surface wave, it is difficult to dispose the reflectors 104, 105 in positions where they have a sufficient reflection characteristic for both the elastic surface waves generated by the driving IDTs 102, 103 and the elastic surface wave caused by the Coriolis force.
If the reflectors 104, 105 are disposed in positions where they satisfactorily reflect the elastic surface waves generated by the driving IDTs 102, 103, the elastic surface waves caused by the Coriolis force are not sufficiently reflected toward the detecting IDT 101 by the reflectors 104, 105. In some cases, the elastic surface wave may be absorbed or reflected in such a manner as to attenuate an incident wave. Thus, it is difficult to detect a voltage corresponding to a sufficient Coriolis force (standing wave) by the detecting IDT 101.
Further, the elastic surface wave gyroscope detects only the Coriolis force by separating it from the elastic surface wave by disposing the teeth of the comb-shaped electrodes D1, D2 of the detecting IDT 101 in the predetermined positions corresponding to the nodes of the standing wave. Accordingly, detection accuracy depends on the accuracy of the mutual positional relationship of the detecting IDT 101, the driving IDTs 102, 103 and the reflectors 104, 105, making it difficult to manufacture a high accuracy or precision elastic surface wave gyroscope.
More specifically, unless the spatial phase difference between the driving IDTs 102, 103 and the detecting IDT 101 is precisely 90.degree., the signal detected by the detecting IDT 101 contains signal components corresponding to the elastic surface waves generated by the driving IDTs 102, 103, causing a detection error.
On the other hand, the position accuracy of the detecting IDT 101, the driving IDTs 102, 103 and the reflectors 104, 105 depends on an electrode fabrication technique on the surface of the piezoelectric substrate. Accordingly, there is a limit to accuracy improvements.
Further, since the elastic surface wave gyroscope is constructed such that the standing waves of the elastic surface waves are generated on the surface of the piezoelectric substrate, and the magnitude of the Coriolis force (standing wave) generated by the interaction of the surface oscillation caused by this standing wave and the rotary motion of the piezoelectric substrate is detected, the magnitude of the standing wave of the elastic surface wave needs to be stabilized in order to stabilize a detection sensitivity with respect to a temperature change.
For example, if the temperature characteristic of an oscillating frequency of a high frequency oscillator for generating high frequency waves to be applied to the driving IDTs 102, 103 and that of a reflection frequency of the reflectors 104, 105 differ, the frequency of the elastic surface waves generated on the piezoelectric substrate 100 and the reflection frequency of the reflectors 104, 105 differ, with the result that the reflection characteristic of the reflectors 104, 105 for the elastic surface waves is reduced. In other words, the phase relationship between progressive waves of the elastic surface waves propagating to the reflectors 104, 105 shifts from a predetermined phase relationship which enables the generation of the standing wave, with the result that the reflectors 104, 105 cannot act to generate a suitable standing wave from the elastic surface waves.
This causes a reduction in the amplitude characteristic of the standing waves of the elastic surface waves and also a reduction in the detection sensitivity of the Coriolis force. The above publication discloses only the construction of the detecting IDT 101, the driving IDTs 102, 103 and the reflectors 104, 105 formed on the surface of the piezoelectric substrate, but does not disclose a method for stabilizing the temperature characteristic of the elastic surface waves to prevent a reduction in the detection sensitivity.